346 CHAPTER 8. MULTICHANNEL SYSTEMS
Figure 8.10: Layout of an integrated four-channel waveguide multiplexer based on Mach–
Zehnder interferometers. (After Ref. [69];©c1988 IEEE; reprinted with permission.)
the wavelength [1]. Demultiplexers based on the MZ filter have attracted the most
attention. Similar to the case of a tunable optical filter, several MZ interferometers
are combined to form a WDM demultiplexer [69]–[71]. A 128-channel multiplexer
fabricated with the silica-waveguide technology was fabricated by 1989 [70]. Figure
8.10 illustrates the basic concept by showing the layout of a four-channel multiplexer.
It consists of three MZ interferometers. One arm of each MZ interferometer is made
longer than the other to provide a wavelength-dependent phase shift between the two
arms. The path-length difference is chosen such that the total input power from two in-
put ports at different wavelengths appears at only one output port. The whole structure
can be fabricated on a silicone substrate using SiO 2 waveguides in the form of a planar
lightwave circuit.
Fiber Bragg gratings can also be used for making all-fiber demultiplexers. In one
approach, a 1×Nfiber coupler is converted into a demultiplexer by forming aphase-
shifted gratingat the end of each output port, opening a narrowband transmission win-
dow (∼ 0 .1 nm) within the stop band [47]. The position of this window is varied by
changing the amount of phase shift so that each arm of the 1×Nfiber coupler transmits
only one channel. The fiber-grating technology can be applied to form Bragg gratings
directly on a planar silica waveguide. This approach has attracted attention since it per-
mits integration of Bragg gratings within planar lightwave circuits. Such gratings were
incorporated in an asymmetric MZ interferometer (unequal arm lengths) resulting in a
compact multiplexer [72].
It is possible to construct multiplexers by using multiple directional couplers. The
basic scheme is similar to that shown in Fig. 8.10 but simpler as MZ interferometers
are not used. Furthermore, an all-fiber multiplexer made by using fiber couplers avoids
coupling losses that occur whenever light is coupled into or out of an optical fiber. A
fused biconical tapercan also be used for making fiber couplers [73]. Multiplexers
based on fiber couplers can be used only when channel spacing is relatively large (>
10 nm) and are thus suitable mostly for coarse WDM applications.
From the standpoint of system design, integrated demultiplexers with low insertion
losses are preferred. An interesting approach uses aphased arrayof optical waveguides